The present invention pertains to thin-film patterning, and more particularly to the patterning of diamond film on a substrate.
Diamond thin films have a variety of uses such as razor blades, tool bits, and surgical instruments, especially in neurosurgery, where diamond thin films have been used in scalpels. Because of its excellent heat conduction properties, diamond thin films have been used in heat-sink applications such as heat conductive coatings in microelectronic components.
A number of methods have been employed for forming a diamond thin film on a substrate in connection with the uses described above. For example, U.S. Pat. No. 4,740,263 discloses the forming of a diamond film on a semiconductor surface using electron assisted chemical vapor deposition (EACVD). In this method, the semiconductor substrate is heated and bombarded with electrons in a hydrocarbon gas to produce nucleation sites on the substrate surface. A similar method is disclosed in U.S. Pat. Nos. 4,830,702 and 4,842,945.
In U.S. Pat. Nos. 4,844,785 and 4,919,974, a diamond film is formed by impinging carbon particles onto a substrate at a high temperature. In U.S. Pat. No. 4,869,923, a nitrogen compound and a carbon compound are placed in a reaction chamber with a semiconductor substrate. The nitrogen compound assists in adhering carbon particles to the substrate surface.
In "Ion-Beam-Assisted Etching of Diamond" by N. N. Efremow, M. W. Geis, D. C. Flanders, G. A. Lincoln, and N. P. Economou, J. Vac. Sci. Technol. B3 (1985), there is a lengthy discussion of an unconventional method of etching a single crystalline diamond layer with xenon and nitrogen dioxide to make desired patterns. Although effective, this technique tends to be cumbersome. Conventional reactive ion etching (RIE) with oxygen, also mentioned in the paper, tends to be slow. As a further complication for polycrystalline diamond, the etching rate differs for each energy for the various faces of the diamond lattice.
One of the more common methods is discussed in Selective Deposition of Diamond Films by J. L. Davidson et al. (Elec. Eng. Dept. of the Alabama Microelectronics Science and Technology Center, Auburn Univ.). The method of this article employs the deposition of diamond films on a "scratched" surface of a silicon substrate. Initially, a polished silicon substrate surface is scratched by a diamond paste. After a cleaning step, a layer of silicon nitride is then formed on the scratched surface and patterned using standard photolithographic processes. Then, exposed silicon is oxidized and the remaining silicon nitride is removed, leaving areas of scratched silicon substrate exposed. A carbon-bearing gas, such as methane, is decomposed near the substrate in a manner which permits carbon radicals of the gas to adhere to the scratched silicon surface, more so than to a smooth silicon surface.
Although scratches in the silicon surface provide nucleation sites for diamond growth, the diamond growth does not always occur. Also, after initially scratching the silicon surface, some particles in the diamond paste are left behind, even after cleaning, forming unwanted nucleation sites. Small particles of any material having a high surface energy can act as nucleation centers. This scratching method is believed to be inadequate for optical substrates and unfeasible for microelectronic substrates which already contain circuitry.
Patterned diamond thin films have been found to be particularly amenable to uses in microelectronic applications. Since diamond structures have such good heat conduction properties, patterned diamond thin films can be used as heat sinks in microelectronic circuitry. Also, non-continuous diamond thin films negate stress components in a semiconductor wafer arising from differential thermal expansion and intrinsic stress.
A problem with the processes noted above is that the forming of diamond thin films on a substrate can not often be accomplished in an economically efficient manner. Also, it is often difficult to form diamond structures in specific areas particularly in a microelectronic environment where space is at a premium.